Liquid such as gasoline stored in an underground tank of a fuel dispensing station is decreased at every dispensing of fuel and eventually by leaking, and increased by resupply made at various times. Liquid such as water is accumulated a little by little for a long time at the bottom of the tank beneath the fuel inevitably containing a very small amount of water, which must be drained when the water level is raised so as not to be mixed with the gasoline supplied to a vehicle.
Thus, the amount of such liquid must be correctly measured in order to confirm the resupplied fuel volume and the total amount of dispensed fuel, as well as for detecting eventual leaking of fuel and avoiding danger of dispensing water mixed with the fuel.
For that purpose an elongated capacitor probe has been widely used in gas stations, which utilizes the fact that water, gasoline or other fuel and gas comprising air and fuel vapor have respectively inherent differential dielectric constants. When detecting electric signals from a number of capacitors, it is possible to determine which segmented electrodes are exposed to water, gasoline and vapor, and consequently detect water level as well as gasoline level. It is also possible to calculate the amount thereof according to geometric principles with reference to the dimensions and configuration of the respective fuel tank. The apparatuses for measuring level and quantity of the liquids provided with such capacitor probe are disclosed in various printed materials, for instance U.S. Pat. Nos. 3,283,577 and 4,349,882.
In such a capacitor probe, the number of the segmented electrodes must be enlarged as far as possible in order to detect correct liquid levels for calculation of correct volumes. The inner electrodes must be formed by assemblying many annular metallic rings with insulating material inserted between every two adjacent rings and arranging wires in order to apply voltage to such many segmented electrodes one by one and collect electric signal respectively from such many electrodes, which inevitably makes the cost of such capacitor probe to be fairly high. In addition thereto, it is difficult for such probe to comply with various fuel tanks of various depths.
Such known capacitor probes have further disadvantages in that when the liquid level happens to lie just between two adjacent electrodes the level detection can not be made and it is not always easy to hold the distance between the inner electrode and the outer electrode to be constant along the considerably extended length of such probe for securing the correct detection of the liquid level.
Furthermore, the electric signals as parameters of electrostatic capacities or dielectric constants of water, gasoline and vapor existing between a number of segmented electrodes and the outer electrode surrounding the inner electrodes are of the nature to be readily attenuated so that correct analysis thereof in the position such as a station cabin remoted more or less from the capacitor probe is difficult for correct calculation of the quantity.